Mass extinctions led to low species diversity, dinosaur rule

IMAGE: The mass extinction at the end of the Permian (about 252 million years ago) was the largest in Earth's history -- 70 percent of land-living vertebrates became extinct. This drastic...
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Credit: Image courtesy of Victor O. Leshyk.

Two of the earth's five mass extinction events - times when more than half of the world's species died - resulted in the survival of a low number of so-called "weedy" species that spread their sameness across the world as the Earth recovered from these dramatic upheavals. The findings could shed light on modern high extinction rates and how biological communities may change in the future.

David J. Button, an NC State and North Carolina Museum of Natural Sciences postdoctoral research scholar, and colleagues examined fossil records of almost 900 vertebrate species dating back between 260 and 175 million years ago - from the late Permian through the Triassic and early Jurassic periods. Two mass extinction events occurred during this time. Button says that similar patterns arising after two mass extinctions implies that other extinction events may have the same results - including current biodiversity loss.

"Mass extinctions not only reduced animal diversity, but also affected the distribution of animals and ecosystems, or biogeography," Button said. "As species are removed by extinction, their ecological niches are left vacant. Following the extinction event, these niches are occupied by surviving and newly evolving 'weedy' species. These few generalists spread out and dominated for a time, leading to a low-diversity global 'disaster fauna.'"

One of these generalists was the Lystrosaurus, a plant-eating early mammal relative that ranged from dog- to pig-sized. It had tusks to help it dig up plant matter.

The late-Permian event - occurring around 252 million years ago - allowed new groups to evolve, including the earliest dinosaurs, crocodiles and relatives of mammals and lizards, Button said. The late-Triassic event, which occurred around 201 million years ago, wiped out many major groups, setting the stage for dinosaurs to take over.

"The late-Permian event caused about 90 percent of sea life and 70 percent of land-living vertebrates to become extinct, probably as a result of climate change from hyperactive volcanism - when volcanoes spewed basalt lava and released gases into the atmosphere causing large increases in carbon dioxide and severe warming resulting in desertification," Button said. "The late-Triassic event is also associated with volcanism."

"Mass extinctions were global disasters that fundamentally reshaped ecosystems," said Richard Butler, professor of palaeobiology at the University of Birmingham and a co-author of the study. "Our new analyses provide crucial data that show just how profoundly these cataclysmic events changed and influenced animal distribution."

"The fossil record has the potential to test evolutionary hypotheses in long time spans, which is not possible if evolutionary research is limited to living plant and animals," said Martín Ezcurra, a researcher at the Museo Argentino de Ciencias Naturales who co-authored the paper.

Identifying patterns across mass extinction events in the fossil record can help researchers make predictions about the consequences of current biodiversity loss, Button said.

"Further understanding of these ancient crises will help to inform conservation efforts to prevent modern animals from suffering a similar fate," he added.

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Graeme T. Lloyd of the University of Leeds also co-authored the paper, which is published in Nature Communications. The study was funded by a Marie Curie Actions grant (630123), a European Research Council Starting Grant (637483) and a Discovery Early Career Researcher (DE140101879) award.

Authors: David Button, NC State University/N.C. Museum of Natural Sciences; Graeme T. Lloyd, University of Leeds; Martin D. Ezcurra, Museo Argentino de Ciencias Naturales; and Richard Butler, University of Birmingham

Published: Oct. 10, 2017 in Nature Communications

DOI: 10.1038/s41467-017-00827-7

Abstract: Mass extinctions have profoundly impacted the evolution of life through not only reducing taxonomic diversity but also reshaping ecosystems and biogeographic patterns. In particular, they are considered to have driven increased biogeographic cosmopolitanism, but quantitative tests of this hypothesis are rare and have not explicitly incorporated information on evolutionary relationships. Here we quantify faunal cosmopolitanism using a phylogenetic network approach for 891 terrestrial vertebrate species spanning the late Permian through
Early Jurassic. This key interval witnessed the Permian-Triassic and Triassic-Jurassic mass extinctions, the onset of fragmentation of the supercontinent Pangaea, and the origins of dinosaurs and many modern vertebrate groups. Our results recover significant increases in global faunal cosmopolitanism following both mass extinctions, driven mainly by new, widespread taxa, leading to homogenous 'disaster faunas.' Cosmopolitanism subsequently declines in post-recovery communities. These shared patterns in both biotic crises suggest that mass extinctions have predictable influences on animal distribution and may shed light on biodiversity loss in extant ecosystems.

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